Researchers at Rensselaer Polytechnic
Institute have developed a new method to harvest
energy from flowing water. Led by Rensselaer Professor Nikhil Koratkar,
the study sought to explain how the flow of water over
surfaces coated with the nanomaterial graphene could
generate small amounts of electricity. Using a small
sheet of the graphene coating, seen above as a dark blue
patch connected to gold contacts, the research team
demonstrated the creation of 85 nanowatts of
power.

Researchers at Rensselaer Polytechnic
Institute have developed a new method to harvest energy
from flowing water. This discovery aims to hasten the creation
of self-powered microsensors for more accurate and
cost-efficient oil exploration.

Led by Rensselaer Professor Nikhil Koratkar, the
researchers investigated how the flow of water over surfaces
coated with the nanomaterial graphene could generate small
amounts of electricity. The research team demonstrated the
creation of 85 nanowatts of power from a sheet of graphene
measuring .03 millimeters by .015 millimeters.

This amount of energy should be sufficient to power tiny
sensors that are introduced into water or other fluids and
pumped down into a potential oil well, Koratkar said. As the
injected water moves through naturally occurring cracks and
crevices deep in the earth, the devices detect the presence of
hydrocarbons and can help uncover hidden pockets of oil and
natural gas. As long as water is flowing over the
graphene-coated devices, they should be able to provide a
reliable source of power. This power is necessary for the
sensors to relay collected data and information back to the
surface.

“It’s impossible to power these microsensors with
conventional batteries, as the sensors are just too small. So
we created a graphene coating that allows us to capture energy
from the movement of water over the sensors,” said Koratkar,
professor in the Department of Mechanical,
Aerospace, and Nuclear Engineering and the Department of Materials
Science and Engineering in the Rensselaer School of Engineering.
“While a similar effect has been observed for carbon nanotubes,
this is the first such study with graphene. The
energy-harvesting capability of graphene was at least an order
of magnitude superior to nanotubes. Moreover, the advantage of
the flexible graphene sheets is that they can be wrapped around
almost any geometry or shape.”

Hydrocarbon exploration is an expensive process that
involves drilling deep down in the earth to detect the presence
of oil or natural gas. Koratkar said oil and gas companies
would like to augment this process by sending out large numbers
of microscale or nanoscale sensors into new and existing drill
wells. These sensors would travel laterally through the earth,
carried by pressurized water pumped into these wells, and into
the network of cracks that exist underneath the earth’s
surface. Oil companies would no longer be limited to vertical
exploration, and the data collected from the sensors would arm
these firms with more information for deciding the best
locations to drill.

The team’s discovery is a potential solution for a key
challenge to realizing these autonomous microsensors, which
will need to be self-powered. By covering the microsensors with
a graphene coating, the sensors can harvest energy as water
flows over the coating.

“We’ll wrap the graphene coating around the sensor, and it
will act as a ‘smart skin’ that serves as a nanofluidic power
generator,” Koratkar said.

Graphene is a single-atom-thick sheet of carbon atoms, which
are arranged like a chain-link fence. For this study,
Koratkar’s team used graphene that was grown by chemical vapor
deposition on a copper substrate and transferred onto silicon
dioxide. The researchers created an experimental water tunnel
apparatus to test the generation of power as water flows over
the graphene at different velocities.

Along with physically demonstrating the ability to generate
85 nanowatts of power from a small fragment of graphene, the
researchers used molecular dynamics simulations to better
understand the physics of this phenomenon. They discovered that
chloride ions present in the water stick to the surface of
graphene. As water flows over the graphene, the friction force
between the water flow and the layer of adsorbed chloride ions
causes the ions to drift along the flow direction. The motion
of these ions drags the free charges present in graphene along
the flow direction — creating an internal current.

This means the graphene coating requires ions to be present
in water to function properly. Therefore, oil exploration
companies would need to add chemicals to the water that is
injected into the well. Koratkar said this is an easy,
inexpensive solution.

For the study, Koratkar’s team also tested the energy
harvested from water flowing over a film of carbon nanotubes.
However, the energy generation and performance was far inferior
to those attained using graphene, he said.

Looking at potential future applications of this new
technology, Koratkar said he could envision self-powered
microrobots or microsubmarines. Another possibility is
harvesting power from a graphene coating on the underside of a
boat.